Semiconductor LEDs have replaced conventional incandescent, fluorescent and halogen light sources in many applications due to their small size, reliability, relatively inexpensive cost, long life and compatibility with other solid state devices. In a conventional LED, an N-type gallium arsenide substrate that is properly doped and joined with a P-type anode will emit light in visible and infrared wavelengths under a forward bias. In general, the brightness of the light given off by an LED is contingent upon the number of photons that are released by the recombination of carriers inside the LED. The higher the forward bias voltage, the larger the current and the larger the number of carriers that recombine. Therefore, the brightness of an LED can be increased by increasing the forward voltage. However due to many limitations, including the ability to dissipate heat, until recently, conventional LEDs have only been capable of producing about six to seven lumens.
Recently, advanced LEDs have been developed which demonstrate higher luminosity, lower power and heat profiles, and smaller footprints enabling the use of multiple LEDs in composite lighting structures. The Cree X-Lamp XR-E, as an example, can produce 136 lumens of luminous flux at 700 mA, with a forward voltage of 3.5V. Its thermal design provides a ratio between the resistance junction and ambient temperature of as low as 13° C./W at maximum current. It provides a small footprint (4.3×7.0×9 mm). They are also reflow-solderable, using a thermal ramp scheme with a 260° C. maximum, enabling certain applications germane to the present invention. Comparable competitive LED products are only slightly behind in market introduction, such as Seoul's Star LED and Luxeon's “Rebel” High Power LEDs.
Such LEDs offer increased brightness over conventional LEDs, and reduce power requirements, but may still suffer from problems associated with heat dissipation and inefficient distribution of light for certain applications. While high-power LEDs, which are a relatively young branch of LED technology, are significantly more efficient than incandescent lights or gas-filled (halogen or fluorescent) lights, they still dissipate on the order of 50% of their energy in heat. If this heat is not managed, it can induce thermal-runaway conditions in the LED, resulting in its failure. Thus, heat management is a critical issue for new applications seeking to take advantage of the efficiencies of LEDs as a source of illumination. For situations requiring high levels of light, this situation is aggravated by the requirement of combining many LEDs in a composite light-source structure.
In marine applications in particular, the use of LEDs as light sources has had limited success to date, awaiting improved efficacy and heat management techniques. Other considerations governing the application of LEDs for marine lighting include the requirement to use wavelengths of light in the range from ˜450 nm to ˜600 nm—blue to green-yellow—for better penetration in a marine underwater environment.